The long-term objective of this R21/R33 investigation is to develop a new nuclear medicine imaging probe for the prostate based on the idea of the Compton scatter camera. When placed close to the organ of interest the probe can achieve high spatial resolution and high counting efficiency simultaneously, and unlike the conventional Compton camera, probes are not significantly influenced by the effects of Doppler broadening. Because of its proximity to the object, the Compton probe Can be small and therefore relatively inexpensive in comparison to a Compton camera for body imaging. In the R21 phase, the possibility for this probe to outperform conventional PET and SPECT instrumentation for prostate imaging will be examined. Performance of the probe and conventional systems for imaging the prostate will be quantified using several measures including the uniform Cramer-Rao bound, the ideal observer, and the channelized Hotelling observer. A prototype probe will be constructed using existing silicon detectors and the C-SPRINT Compton camera. Statistics from phantom measurements using the probe will be used to corroborate predictions of the performance measures. ff the investigation indicates that the probe provides significantly better images of the prostate, full-scale development of the Compton probe and preliminary patient imaging studies will be conducted in the R33 phase. Encouraging results in R33 will stimulate commercialization.